Community-acquired pneumonia (CAP) is the most common cause of acute respiratory distress syndrome (ARDS), a severe form of acute lung injury that is one of the most frequent causes of admission into the intensive care unit. Few therapeutic options are available, and mortality is high. Supportive therapy with supplemental oxygen and mechanical ventilation are essential, but additional injury can be caused by the ventilator, termed ventilator-induced lung injury (VILI). Epithelial repair is critical for disease resolution and survival, but we have limited knowledge of the underlying mechanisms of repair and how mechanical stretch impacts these mechanisms. The long term objective of this project is to increase our understanding of the mechanisms of epithelial repair and how overdistention of pulmonary epithelial cells contributes to VILI and maladaptive repair mechanisms. We previously identified an autocrine role for the chemokine CXCL12 in alveolar epithelial repair involving its receptor CXCR4. We now have preliminary data showing that patients with CAP-induced ARDS that had high levels of CXCL12 in their bronchoalveolar lavage fluid had shorter duration of mechanical ventilation and lower mortality. Based upon additional preliminary data, we propose that CXCR4 interacts with a complex of signaling molecules including focal adhesion kinase (FAK) and apoptosis signal-regulating kinase-1 (ASK1) that regulates epithelial repair. The central hypothesis of this application is that CXCL12 promotes epithelial repair, but mechanical stretch causes disruption of CXCR4-FAK-ASK1 signaling that inhibits cell spreading, migration, and repair. We will first examine whether CXCL12 is a biomarker for ARDS patients undergoing adaptive repair by measuring CXCL12 in banked samples of bronchoalveolar lavage fluid and plasma. In addition we will use autopsy samples from ARDS patients to evaluate expression of CXCR4 and phosphorylated (activated) ASK1. In the second aim we will investigate the interactions between CXCR4, FAK, and ASK1 during recovery from lung injury caused by LPS as a model of pneumonia. We will use mice with conditional deletion of CXCR4 in lung epithelial as well as ASK1 knockout mice. We will also examine the biochemical interactions of these signaling molecules in cultured alveolar epithelial cells in a scratch wound model. In the third aim we will investigate how high stretch mechanical ventilation or cyclic stretch of cultured cells disrupts these signaling pathways during repair in combined model of pneumonia (LPS) and mechanical ventilation. These studies will elucidate new signaling pathways involved in alveolar epithelial repair and how mechanical stretch disrupts the repair processes.